Electromechanical oscillators



May 1963 J. A. CUNNlNGHAM 3,091,151

ELECTROMECHANICAL OSCILLATORS Filed Nov. 18. 1960 W9 7 5 I 25 faINVENTOR.

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" atenr 3,091,151 Patented May 28, 1963 iiice 3,091,151ELECTROMEC-HANICAL OSQILLATORS John A. Cunningham, Batavia Township,Kane County, Ill. (Rte. 1, Box 7, Batavia, Ill.) Filed Nov. 18, 1960,Ser. No. 70,190 2 Claims. (Cl. 84-457) This invention relates toimprovements in electromechanical oscillators and more particularly tothe novel construction and assembly of :a unit wherein the vibratingelement will oscillate at a constant frequency irrespective oftemperature variations.

In many applications, mechanical oscillators, such as tuning forks, areemployed to generate an alternating current of substantially constantfrequency and usually are driven by electro-magnetic means. Thevibrating frequency of tuning forks changes with variations in thetemperature of the tuning fork and its associated magnets. The maincauses for such variations are the changes that occur in the dimensionsand elasticity of the tines of the fork and changes in the strength ofthe magnetic fields of the magnets. An increase in length, a decrease inelasticity or a decrease in the strength of the magnetic field willresult in a lowering of the pitch or vibration frequency of the tuningfork tines; whereas a decrease in its length, increase in its elasticityor an increase in the strength of the magnetic field will raise thepitch or vibration frequency.

Any increase or decrease in temperature is directly related to themagnetic influence from the electro-magnets with which the times areassociated. When the air gap between the pole pieces of theelectromagnetic driving means and the free end of the tuning fork tinesis adjusted to a certain critical distance, a constant vibrationfrequency irrespective of temperature is obtained. Thus, if the air gapbetween the electro-magnet and tuning fork tine is greater than thecritical distance, an increase in temperature will result in an increasein the frequency (a positive temperature vs. frequency coefficient).However, if the air gap is less than the critical distance, an increasein temperature will result in a decrease in frequency (a negativetemperature vs. frequency coefiicient). Once the critical air gap isdetermined and the adjustment locked in place, changes in temperature nolonger result in changes in frequency, and a zero temperature vs.frequency coeflicient is obtained.

The representative oscillator herein disclosed is characterized by theinclusion of a resonator in any accep table form, such as a tuning fork,disposed in association with electro-magn'et driving and pick-up means,which means are adjustable, or at least the magnetic cores of which areadjustable, relative to the tines, so as to vary the air gap between thetines and cores to thereby control the induced eddy currents by a changeof the magnetic influence of the electromagnetic means and alter thetemperature vs. frequency coeflicient.

Fine tuning or minute influencing of the temperature coefficient isaccomplished herein by the novel construction of the electromagneticdriving and pickup means, or the cores thereof, which are independentlyor jointly adjustable to vary the air gaps between the driven andpick-up faces of the tines and the influencing magnetic cores. Morespecifically, if a tuning fork answers a prescribed temperaturecoefiicient versus frequency test and measurements of a specific value,this value can be changed by use of the herein disclosed structure ineither a positive or a negative direction from zero.

When the air gap is greater than the critical gap, a reduction in theair gap between the tines and the electromagnetic means will increasethe eddy currents and influence the temperature coeiiicient value in apositive direction causing the temperature coefiicient versus frequencycurve to either come closer to zero from a negative area or to gofurther from zero in a positive direction to a more positive area. Apositive coeflicient of fre: quency versus temperature will cause thetines to change frequency by an increase of frequency with an increaseof temperature.

When the air gap is less than the critical gap, an increase in the airgap and the resulting decrease of the eddy currents will change thetemperature coefficient value in a negative direction causing thetemperature coefficient versus frequency curve to either move from thezero area to a more negative area or to approach more closely the zeroarea from a positive area and become less positive. A negativecoefficient of frequency versus temperature will cause the tines tochange frequency by a reduction or lowering of frequency with anincreased temperature.

The herein disclosed resonator is structurally designed to assist inproducing an effective zero coefficient of frequency versus temperaturefor the resonator and it is one of the objects of the invention toprovide such a device.

Another object is to provide an electromechanical oscillator whereinmicrometric adjustment may be made in the gap between the electro magnetcore and the tines of the resonator.

With the foregoing and such other objects and advantages in view, whichwill appear as the description proceeds, the invention consists ofcertain novel features of construction, arrangement and combination ofparts hereinafter fully described, illustrated in the accompanyingdrawings, and particularly pointed out in the appended claims, it beingunderstood that various changes in form, proportion, size and minordetails may be made without illustrating the principles of theinvention.

Referring to the drawings in which the same characters of reference areemployed to identify corresponding parts:

FIG. 1 is a plan view of a representative oscillator illustrating theprincipals of the invention.

FIG. 2 is an enlarged transverse sectional view taken on line 22 of FIG.1.

FIG. 3 is a graphic representation of a curve showing a negativecoefficient of temperature versus frequency.

FIG. 4 is a graphic representation of a curve showing a positivecoefficient of temperature versus frequency.

FIG. 5 is a graphic representation of a constant frequency.

Referring to the exemplary embodiment of the invention disclosed in theaccompanying drawings, the electromechanical oscillator shown in FIG. 1comprises a base or mounting block 11 longitudinally slotted inwardlyfrom one end, as at 12, and recessed on its top face at the unslottedend to receive a resonator or temperature compensated tuning fork 13,the tines 14 of which extend into slot 12. Screws or rivets 15 securethe base end of the tuning fork in the recess.

A pair of upstanding bosses 16 are located laterally one on each side ofslot 12. Each of these bosses mounts an electro-magnet assembly,generally indicated at 17, so arranged that the core 18 of each has oneof its ends of poles spaced closely to the respective driven and pick upfaces of tines 14 of the tuning fork.

As is well understood in this art, an increase in the gap between thecores 18 and the tines 14 will increase the frequency of the tuning forkoscillations, whereas a de crease in the gap will reduce the frequencyof the tuning fork oscillations. Consequently, each electro magnet ismounted in the boss for adjustment toward and away from the tuning forkso as to adapt the assembly for adjustment to alter the size of the gapsbetween their cores and the respective tines in order to arrive at thecritical air gap adjustment for a zero temperature frequencycoefficient.

Referring now particularly to FIG. 2, each magnetic core 18 preferablyhas its mounted end screw threaded, as at 21, for threading it throughits mounting boss so as to be adjusted axially upon being rotated ineither direction. Preferably, the threads are generated with precisionso that micrometric adjustment can be attained and determined byrotating the cores a predetermined amount. For example, by providingforty (40) threads per inch, there will be a change of .025 inch in theposition of the core for each rotation. Obviously, the electromagneticcoils 22 may be mounted upon the magnetic cores in any conventionalmanner. The axial adjustment once determined is fixed by set screws 26in the top of the bosses 16.

As previously discussed, if the oscillating frequency of the tuning islowered by an increase in its temperature it is said to have a negativecoefficient of temperature versus frequency. Also, if this same increaseof temperature causes the frequency of the tuning fork to rise, the forkis said to have a positive coefficient of temperature versus frequency.

Either effect causes a change in the rate of oscillating frequency, acondition that is obviated by the present arrangement which allows thegap between the electro magnet cores and the opposed surfaces of thetuning fork tines to be adjusted to the critical air gap. This canperhaps be best illustrated by the following test results.

A mechanical oscillator was caused to operate at a given temperature andthe oscillating frequency of the tines was recorded as was also the gapbetween the tines and the cores. With these conditions remainingconstant, the rate of frequency change resulting from heating of thetuning fork was recorded in either a positive or a negative direction.In the instant test, the tuning fork was determined to have a negativecoeflicient of 100 parts per million over a temperature range of C. toplus 100 C. This is a frequency change of .001 cycle per second perdegree centigrade. In FIG. 3, the line 23 is representative of thiscondition.

The gaps between the tines and the cores were decreased by .001 of aninch and the test repeated. This produced a positive coefiicient oftemperature versus frequency of the same rate as was present in thenegative 4 direction, but in the positive direction. This is illustratedby line 24 in FIG. 4.

Readjustment of the gaps was continued until the critical gaps wereestablished that produced a zero coefficient of temperature versusfrequency, although the temperature of the tuning fork ranged from 0 C.to plus C. With these temperature variations, the oscillating frequencyremained constant, as illustrated by line 25 in FIG. 5, within thelimits of employed audible frequency determinating equipment andtemperature measuring equipment.

As many possible embodiments may be made in the invention, and as manychanges might be made in the embodiment selected forillustration, it isto be understood that all matters hereinbefore set forth and shown areto be interpreted as illustrative and not in a limiting sense.

What I claim and desire to secure by Letters Patent of the United Statesis:

-1. In combination with a tuning fork, single pole electromagnetcontrolling the vibration of said fork, the pole faces of saidelectromagnets being located near the free ends of the tines of saidtuning fork, and means to adjust the pole faces of the electromagnetstoward and away from the tines to obtain the critical air gap tomaintain an effective zero coefficiency of frequency versus temperaturefor the tuning fork.

2. In combination with a tuning fork having a given coefiicient ofelasticity, an electromagnet controlling the vibrations of said tuningfork, said electro magnet comprising a single core having an electriccoil therearound, the core of said electrom-agnet having its magneticpole spaced from one of the tines of said fork, a mounting for saidelectro magnet including a boss, and said core being threaded into saidboss so as to be adjustable toward and away from the tines to decreaseor increase the gap between its pole and the time and control the eddycurrent loss for influencing the temperature coefficient.

References Cited in the file of this patent UNITED STATES PATENTS1,637,442 Dorsey Aug. 2, 1927 2,384,823 Eisenhour Sept. 18, 19452,928,308 Godbey Mar. 15, 1960 FOREIGN PATENTS 636,563 Great Britain May3, 1950

1. IN COMBINATION WITH A TUNING FORK, SINGLE POLE ELECTROMAGNETCONTROLLING THE VIBRATION OF SAID FORK, THE POLE FACES OF SAIDELECTROMAGNETS BEING LOCATED NEAR THE FREE ENDS OF THE TIMES OF SAIDTUNING FORK, AND MEANS TO ADJUST THE POLE FACES OF THE ELECTROMAGNETSTOWARD AND AWAY FROM THE TIMES TO OBTAIN THE CRITICAL AIR GAP TOMAINTAIN AN EFFECTIVE ZERO COEFFICIENCY OF FREQUENCY VERSUS TEMPERATUREFOR THE TUNING FORK.